DE69706799T2 - White foils, suitable for lamination on metal surfaces and processes for their production - Google Patents

Description

FIELD OF INVENTION

The present invention relates to a white foil or a white film for laminating a metal surface and a method for its production.

BACKGROUND OF THE INVENTION

Metal cans, such as steel cans and aluminum cans, used as packaging containers for food and drinks, are in large number of use. To provide these metal cans with corrosion resistance and printability, solvent-based paints, mainly consisting of heat-set resins, have been applied to the can surfaces.

However, the use of such paints reduces the output and causes problems including environmental pollution. Therefore, in recent years, laminates made as laminated biaxially stretched plastic films or as heat-sealed layers have been increasingly used, wherein such a plastic film is used as a substrate to provide a laminated film which is then laminated to a metal strip or metal plate and subjected to various deformations. and processing techniques for use as a metal can.

Metal cans covered with a plastic film are manufactured by laminating a plastic film onto a metal strip (including those that are plated or otherwise surface-treated), such as a steel strip or aluminium strip, and subjecting this to a forming process. Plastic films for such use must simultaneously satisfy the following various properties:

(i) Improved laminability of the metal strips.

(ii) Improved can formability. That is, the film endures the can forming process without peeling of the layers, without cracking or tearing and without pinhole formation.

(iii) No possibility of destroying the taste of the contents of the can (if the plastic film is used on the inside of the can).

(iv) Retort processing shall not result in occurrence of water spots or white powder. (Water spots means a phenomenon in which a film which has melted and not crystallized during lamination absorbs water drops to crystallize and turn white during retort processing. If water spots occur, it degrades the aesthetic appearance of the goods. White powder means a type of low molecular weight substance such as an oligomer which is deposited on the film surface. If a laminated film is used on the inside of a can, the white powder degrades the taste of the contents of the can, while if it is used on the outside of the can, the aesthetic appearance of the can is degraded by the white powder.)

Usually, cans are printed on the outside, whereby a white paint is applied as a base coat to conceal the metallic paint and then the print is applied on top. In recent years, in order to simplify the manufacturing processes (reducing energy and costs) and to address environmental problems (no use of solvents), the production of cans with a white film laminated on top to conceal the paint has been developed.

As for a white film, use is made of a polyester resin mixed with a high concentration of titanium oxide. However, such a material is not sufficient for whiteness and concealability, and it has been desired to increase the content of titanium. Increasing the amount of titanium, however, poses problems including stiffening of the film surface, wearing out of the metal press or die for producing the cans, and adhesion of metal chips or titanium oxide to the film surface, resulting in uneven printing during printing and making laminability difficult with respect to steel strips.

As for such a white film for metal lamination, one is disclosed in EP-A-0 717 066, which is a biaxially stretched film having first and second layers, at least one of the first and second layers being composed of polyester and titanium oxide. Specifically, the first layer is made of a polyester group resin containing a white pigment having an average particle diameter of 0.01 to 1.8 µm at a content of 10 to 30% by weight, the first layer having a void ratio of 4 to 30%, a melting peak temperature of 150 to 245°C, and an amount of oligomer "OB" of not more than 3% by weight and an orientation degree of 0.7 to 4.0. The second layer is laminated on the first layer and is made of a polyester group resin containing a white pigment having a content of 10 to 30% by weight, the second layer having an amount of oligomer "OA" (in weight %) that satisfies the equation OA < (OB-0.2). The white pigment of the first and second layers includes titanium oxide particles having an average particle diameter of 0.1 to 1.2 µm and a drying loss of not more than 0.8%. Specifically, the titanium oxide particles are rutile-type titanium oxide particles treated with a silane or an alcohol.

Another white film for metal lamination disclosed in Japanese Patent Laid-Open No. 5-170942 is prepared by blending titanium oxide into copolymer polyester to improve can formability. Further, another is disclosed in Japanese Patent Laid-Open No. 5-339391 which is prepared by blending rutile-based titanium oxide of 95% or greater purity into a copolymerized polyester. Furthermore, another is disclosed in Japanese Patent Laid-Open No. 6-271686 which is prepared by blending master chips of titanium oxide of high concentration and a diluted polymer having a wide viscosity range to improve can formability and impact resistance. Further, another one is disclosed in Japanese Patent Laid-Open No. 6-49234, which is prepared by mixing diluted polymer of high viscosity with granules of titanium oxide to improve impact resistance. Furthermore, a laminated polyester film prepared by laminating two copolymerized polyesters having different pigment concentrations is disclosed in Japanese Patent Laid-Open Nos. 6-39980 and 7-52351.

Although single-layer or double-layer films containing titanium oxide filled in polyester resin have been proposed as described above, none of these meet two requirements at the same time; improvement in whiteness and reduction in press wear. Die for the production of cans.

In addition, in recent years, the desire to improve the whiteness and concealability of can films has been joined by the desire to reduce the thickness of films to reduce costs. To meet this desire, it is necessary to mix titanium oxide at a higher filling density.

But the incorporation of titanium oxide with a higher filling density poses several problems; (a) during the production of films, partly during stretching, cracking must be expected, which reduces the functionality, (b) lamination on metal strips is difficult, (c) during the molding process of film-laminated metal strips, presses made of metal or similar tend to be destroyed, (d) it is difficult to print on laminated film surfaces (errors must be expected).

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a white film for laminating to metal surfaces and a method for producing the same, which white film is superior in heat adhesion (heat laminability) to metal strips, formability into cans, concealability and whiteness and which does not cause damage to the press during production of cans, these features making the white film suitable for use as a coating of metal cans.

By using polyesters of special chemical composition as raw materials to produce a film in three-layer construction, which has three layers of different materials, with a first layer in the middle and second layers, front and back, we have found that the above problems can be solved, that a film having high whiteness and high concealability can be obtained, and that the processing efficiency during production of the film can be significantly improved by using different amounts of titanium oxide incorporated in the first and second layers, that is, locally setting the amount of titanium oxide in the first layer; thus, we have achieved the present invention.

According to the invention, the white film has better heat laminability, formability and stretchability and is better in whiteness and concealability and also in printability, these properties make the white film optimal for use as an outer coating for metal cans, the white film causing less wear of the metal press or die for producing metal cans.

More detailed, a white film suitable for lamination on a metal surface according to the present invention comprises first and second layers laminated in the arrangement second layer/first layer/second layer, wherein

- the first layer is made of a composition consisting of 80 to 40% by weight of polyester, which in turn is composed of 100 to 75 mol% of ethylene terephthalate unit and 0 to 25 mol% of ethylene isophthalate unit, these units add up to 100 mol%, and 20 to 60% by weight of titanium oxide, which is mixed with the polyester so that the total adds up to 100% by weight, the second front and back layers are made of a composition consisting of not less than 80 to 100% by weight of polyester, which in turn is composed of 94 to 70 mol% of ethylene terephthalate unit, 5 to 25 mol% of ethylene isophthalate unit and 1 to 5 mol% of diethylene tere(iso)phthalate unit, these units add up to is composed of 100 mol% and 20 to 0 weight% titanium oxide, which is mixed with the polyester so that the total adds up to 100 weight%,

- wherein the film is a biaxially stretched film,

- the amount of titanium oxide contained in the film is 20 to 50% by weight.

A method of producing a white film for laminating to a metal surface according to the invention is a method in which the white film comprises first and second layers laminated in the arrangement of second layer/first layer/second layer, wherein

- the first layer is made of a composition consisting of 80 to 40% by weight of polyester, which in turn is composed of 100 to 75 mol% of ethylene terephthalate unit and 0 to 25 mol% of ethylene isophthalate unit, these units adding up to 100 mol%, and 20 to 60% by weight of titanium oxide, which is mixed with the polyester so that the total adds up to 100% by weight, the second front and back layers are made of a composition consisting of not less than 80 to 100% by weight of polyester, which in turn is composed of 94 to 70 mol% of ethylene terephthalate unit, 5 to 25 mol% of ethylene isophthalate unit and 1 to 5 mol-% diethylene tere(iso)phthalate unit, these units add up to 100 mol-%, and 20 to 0 weight-% titanium oxide, which is mixed with the polyester so that the total adds up to 100 weight-%,

- wherein the film is a biaxially stretched film, the amount of titanium oxide contained in the film is 20 to 50% by weight,

the procedure includes the following steps:

- use of resins such that the inherent viscosities of the polyesters in the first and second layers are 0.5 or more;

producing an unstretched strip by molten coextrusion of these resins; and

- Stretching the resulting unstretched strip longitudinally and transversely.

Therefore, according to the invention, there is provided an inexpensive multilayer white film for metal lamination and a method for producing the same, the white film having superior heat laminability (heat adhesion), formability and strength and being superior in whiteness and concealability, so that it can be suitably used for coating metal cans.

The present invention will now be described in more detail.

A white film of the invention is a polyester film comprising an intermediate layer having a high titanium oxide concentration (hereinafter referred to as the B layer) and layers having a low titanium oxide concentration arranged on the opposite sides of the intermediate layer (hereinafter referred to as the S layer). The S layers are made of a material consisting of 94 to 70 mol% of ethylene terephthalate unit, 5 to 25 mol% of ethylene isophthalate unit and 1 to 5 mol% of diethylene tere(iso)phthalate unit, these units adding up to 100 mol%. The B layer is made of a material consisting of 100 to 75 mol% of ethylene terephthalate unit and 0 to 25 mol% of ethylene isophthalate unit, these units adding up to 100 mol%.

The content of ethylene isophthalate in the S-layers is 5 to 25 mol% as described above, but preferably 8 to 15 mol%. If it is less than 5 mol%, the heat laminability between the film and the metal, such as a steel strip, and the processability of a film-laminated steel strip into cans is reduced. If it is larger than 25 mol%, it would bring problems in heat stability during heat application and in basic post-processing after the production of cans; the material loses crystallizability, it becomes difficult to dry the resin pellets sufficiently, problems occur in film production, the film becomes non-crystallizable, causing a lack of film strength and heat stability, and the film-laminated metal strip is caught on the hot roller during the can-making process.

The proportion of diethylene tere(iso)phthalate unit in the S-layers is 1 to 5 mol% as described above, but preferably 2 to 4 mol%. The introduction of diethylene tere(iso)phthalate unit in this proportion range as a polyester component makes it possible to adjust the balance between the non-crystallizability causing effect caused by the ethylene isophthalate unit and the crystallizability effect of the ethylene terephthalate unit, so as to provide heat laminability between the film and the metal, cannability, heat stability and strength.

Further, in terms of strength, heat stability and heat laminatability, it is advantageous that the sum of the ethylene terephthalate unit and the diethylene tere(iso)phthalate unit in the S layers is in a range of 6 to 25 mol%. If it is less than 6 mol%, it lowers the heat laminatability between the polyester film and the metal, and the manufacturability of film-laminated steel strips into cans. If it exceeds 25 mol%, it would lower the crystallizability, making it difficult to dry the pellets sufficiently. Further, the film strength and heat stability would be incomplete or the film would not be crystallizable, resulting in wrapping the hot roll by the film-laminated metal strip during the can production process.

The content of the ethylene isophthalate unit in the B layer is 0 to 25 mol% as described above, but preferably 5 to 15 mol%. If 25 mol% is exceeded, the material would lose its crystallizability, making it difficult to dry the pellets sufficiently. Further, the film would not be crystallizable, resulting in a lack of strength and heat stability, or the film-laminated metal strip would tend to wrap around the hot roll during the can-making process.

It is possible to control the balance of crystallizability of the film by copolymerizing the polyester in the B layer with less than 5 mol%, preferably 1 to 4 mol%, of diethylene tere(iso)phthalate unit.

The polyesters used in the present invention may be single polyesters or two or more melted and blended polyesters.

The polyester materials for the S and B layers should have an inherent viscosity of 0.5 or more, preferably 0.6 to 1.2. Using a polyester with an inherent viscosity of less than 0.5 would reduce the processability during film production and the resulting film would lack strength. However, very high inherent viscosity is also not recommended, as this would significantly increase production costs.

It is desirable that the difference in inherent viscosity between the polyesters in the S and B layers is between 0.1 or less, preferably 0.09 or less. If the difference in inherent viscosity exceeds 0.1, this would result in a flow mark in the film during the film manufacturing process. A flow mark is meant as a kind of resin flow change, ie a flow pattern of the resins which is formed when two or more resins of different viscosities that flow meet in a feed block or a T-die.

The polyesters used in the invention are preferably those having a melting point of 200 to 240°C if they are crystalline or a glass transition temperature of 50 to 85°C if they are not crystalline. Deviation from these ranges would result in insufficient heat stability or in decreasing heat laminability.

The polyesters forming the S and B layers can be optionally combined within the range that satisfies the conditions. Among all the possible combinations, one is particularly advantageous in which the polyester in the S layers has a melting point (or glass transition temperature if it is not crystalline) equal to or lower than that of the polyester in the B layer. This makes it possible to provide a good balance between the strength of the heat stability and the laminability of the film.

The polyesters used in the invention may have components other than isophthalic acids and diethylene glycol which copolymerize within the range without destroying the characteristics. Examples of such copolymeric components are dicarboxylic acids such as phthalic acid, 2,6-naphthalene dicarboxylic acid, 5-sodium sulfoisophthalic acid, oxalic acid, succinic acid, adipic acid, sebacic acid, azelaic acid, dodecanoic acid, dimer acid, maleic anhydride, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid and cyclohexanedicarboxylic acid. Other examples are 4-hydroxybenzene acid and oxycarboxylic acids such as ε-caprolactone and lactic acid. Still other examples are glycols such as e.g. B. 1,3-propanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, cyclohexanedimethanol, triethylene glycol, polyethylene glycol, polypropylene glycol, polythetramethylene glycol and ethylene oxide addition products of bisphenol A and bisphenol S. Furthermore, minor use may be made of compositions having three functional groups, such as trimellitic acid, trimesic acid, pyromellitic acid, trimethylolpropane, glycerin and pentaerythritol.

The content of titanium oxide in the film of the present invention, represented as the average concentration in the film, is 20 to 50% by weight, preferably 25 to 45% by weight, most preferably 25 to 40% by weight.

If the content is less than 20% by weight, the resulting film would be deficient in whiteness and concealability. If the content exceeds 50% by weight, the resulting film, even if it has a three-layer construction as in the present invention, would be decreased in strength and inferior in formability after lamination.

The content of titanium oxide in the B layer is 20 to 60% by weight, preferably 25 to 55% by weight, most preferably 30 to 50% by weight. If the content is less than 20% by weight, the resulting film would show a lack of whiteness and concealability. If the content exceeds 60% by weight, the resulting film would decrease in strength and be inferior in formability after lamination.

The content of titanium oxide in the S layers, under the condition of including 0 wt%, is not more than 20 wt%, preferably not more than 15 wt%, most preferably 10 wt%. If the content exceeds 20%, it would sometimes result in press wear during lamination to the metal strips or during production of cans from the film-laminated metal strips, which proves to be a considerable adverse influence on the manufacturing process, or cause particles of the scraped metal or titanium oxide to stick on the Film surface during the can making process, which poses a problem regarding printability.

A known single-layer white film having a titanium oxide content of about 20% by weight has insufficient whiteness and concealability. If, as a countermeasure, the titanium content is increased to increase the whiteness and concealability, it brings a problem caused by the wear of the press during can production, a problem that particles of the scraped metal or titanium adhere to the film surface to cause an uneven print image during printing, and a problem of difficulty in heat lamination with respect to steel strips.

In contrast, the film of the present invention is capable of solving such various problems by lowering the concentration of titanium oxide in the S layers which are the outer layers and by raising the concentration of titanium oxide in the B layer which is the inner layer.

According to the present invention, the above various problems can be solved more effectively by making the ratio Cb/Cs of the concentration Cb (in weight %) of titanium oxide mixed in the B layer to the concentration Cs (in weight %) of titanium oxide mixed in the S layers 3 or more, preferably 5 or more.

Ts is the thickness of the S layer (in µm) and Tb is the thickness of the B layer (in µm). If the relationship between the titanium oxide concentrations in the S and B layers and the thickness of each layer satisfies the following equation

3 ? (Cb/Cs) · [Tb/(Tb+Ts)]

it is possible to improve the whiteness and concealability while maintaining the mechanical characteristics and formability of the film. That is, increasing the titanium oxide concentration Tb of the B layer, which is the inner film layer, relative to the titanium concentration of the S layers, which are the outer layers, makes it possible to decrease the thickness Tb of the inner film layer while keeping the whiteness and concealability the same. If the value of the right-hand side of the above formula is less than 3, it is necessary to increase the total thickness of the film before the whiteness and concealability of the film can be improved. This is not only responsible for a decrease in formability but also for a decrease in economic efficiency.

Titanium oxide can be used subsequently for any known surface treatment, depending on the requirement. Furthermore, titanium oxide can be processed in a mixer to provide a master batch containing 50 to 70% by weight of titanium oxide, or it can be added directly during the film production process.

It is preferable that the titanium oxide has an average particle size of 0.1 to 0.5 µm, preferably 0.2 to 0.5 µm. If it is larger than 0.5 µm, the total surface area per unit weight of titanium oxide would decrease, sometimes resulting in a lack of concealability and whiteness of the film. Furthermore, the resulting film surface would be uneven and lose gloss, making it unsuitable for printing. If it is smaller than 0.1 µm, the average particle size would be smaller than the wavelength of visible rays, so that the latter rays can pass through the film. Also, the film would have a lack of concealability and whiteness.

It is preferable that the films of the present invention satisfy the following characteristics 1 to 4.

(1) The tensile stress is not less than 98 MPa (10 kgf/mm²), preferably 127 MPa (13 kgf/mm²).

(2) The longitudinal (MD) and transverse (TD) heat shrinkage factors of the film at 150ºC, 30 minutes is not more than 6%, preferably 0.5 to 5.0%.

(3) The optical density is not less than 0.3, preferably 0.4 to 0.7.

(4) The whiteness is not less than 81.0, preferably 85.0.

If the tensile stress is less than 98 MPa (10 kgf/mm2), the working strength would be insufficient. If the thermal shrinkage factor is less than 6%, adhesion to steel strips would be reduced. If the optical density is less than 0.3, the concealability would be insufficient. If the whiteness is less than 81.0, the whiteness would be insufficient for use.

If the lower limits of these characteristic values are defined, the larger the values are, the better, while if the upper limits are defined, the smaller the values are, the better. However, excessively high quality increases costs and also reduces workability during the film production process; therefore, the above ranges are appropriate.

The thickness of the B layer is 5 to 20 µm, preferably 10 to 15 µm, while the thickness of each S layer is 0.5 to 5 µm, preferably 1 to 3 µm, the total thickness is preferably 6 to 30 µm. If the thickness of the B layer is less than 0.5 µm, the film would be insufficient in terms of width and concealability, and if it exceeds 20 µm, it would cause excessively high quality, thereby losing cost performance. Furthermore, if the thickness of the S layer is less than 5 µm, it would result in wear of the press during the can making process or a reduction in printability. On the other hand, if it is greater than 5 µm, the mechanical property of the films would be improved, but the whiteness and concealability would not be so good at this film thickness and delamination would likely occur at the interface between the S and B layers during the can making process. The total film thickness is preferably 9 to 25 µm, most preferably 12 to 17 µm, to ensure formability for drawing and smoothing operations.

The heat shrinkage factor of the films of this invention at 150ºC, 30 minutes is preferably such that

TD ≤ 6%, MD - TD ≥ 3%

most preferably so that

TD ≤ 5%, MD - TD ≥ 35%

to prevent unevenness during heat lamination on steel strips.

Polyesters for films according to the invention can be prepared in a conventional manner. For example, copolymers of the polyethylene terephthalate type in which isophthalic acid components and diethylene glycol are copolymerized and prepared as follows.

First, a slurry or mixture of terephthalic acid and ethylene glycol is continuously added to an esterification vessel containing bis(β-hydroxyethyl) terephthalate and/or a lower polymer thereof, and which are allowed to react with each other at 250ºC for about 8 hours, thereby continuously producing an ester having an esterification percentage of about 95. This product is then transferred to a polymerization vessel where a required amount of isophthalic acid or ethylene glycol ester thereof and diethylene glycol are added. And in the presence of a catalyst such as antimony trioxide or germanium dioxide, polycondensation is initiated at a reduced pressure of not greater than 1.3 hPa at about 280ºC.

The film of the invention can be produced by the following methods.

Unstretched strips are produced by a method using separate extruders so that two resin compositions constituting the individual layers are extruded after being melted and superimposed one over the other by the feeding process through a press, a method comprising the step of placing two resin compositions constituting the individual layers on top of each other in a multi-press and extruding them, or by a combination of these two methods. Then, such an unstretched strip is stretched longitudinally and transversely using the biaxial tension-stretching process or the inflation process, thereby obtaining the film of the invention.

Alternatively, it is possible to use a process in which two types of stretched films forming the individual layers are doubled.

When the biaxial tension-stretching process is used, two types of polyester compositions mixed with titanium oxide, which make up the S and B layers, are added to a melt extruder from which they are extruded into a strip form at a temperature of 220 to 280ºC, the extruded strip is contacted with a cooling drum for cooling which is maintained at a controlled temperature below room temperature.

The unstretched strip is then preliminarily stretched 1 to 1.2 times in the MD depending on use, and thereafter it is stretched biaxially with a tenter at a temperature of 50 to 150ºC with a stretch magnification factor of 2 to 4 times in both MD and TD. It is then subjected to a heat treatment at 80 to 220ºC for a few seconds to provide some percent relaxation in the TD, thereby producing the film of the invention.

The biaxial stretching processes using a tensioner include the simultaneous biaxial stretching process and the sequential biaxial stretching process. If a larger content of titanium oxide is used, the film will tend to break easily during the stretching process; however, the use of the simultaneous stretching process reduces the occurrence of such breaks with a good result. Therefore, the simultaneous stretching process is more suitable.

The heat treatment after the stretching process is a process to reduce the heat shrinkage factor of the film. Known heat treatment methods can be used, such as the one that blows hot air, another that emits infrared rays, and another that emits microwaves. Of these methods, the one that blows hot air is the most suitable because of the ability of uniform and accurate heating of the film.

In order to facilitate the transition from process to process, such as the transition to the film-making process or the can-making process, it is desirable to add a small amount of inorganic lubricants such as silicon, aluminum or kaolin to form a film to which lubricity is imparted on the film surface. Furthermore, in order to improve the external appearance and printability of the film, it is also possible to add a silicone component or the like to the film.

Furthermore, to improve laminability to metal or to increase strength, a coating layer, such as an adhesive layer, can be applied by a run-on coating during film formation or by a subsequent coating after film formation.

In cases where the metal strip to be laminated with the film of the invention is a steel strip, it is preferable to use a steel strip that has been subjected to a chemical treatment such as a chromic acid treatment, a phosphoric acid treatment, an electrolytic chromium treatment or a chromate treatment or a steel strip plated with nickel, tin, zinc, aluminum, gun metal, brass or the like.

The invention will now be described concretely with reference to the examples.

In the following examples and comparative examples, the characteristic values of the film are measured as follows.

A. Inherent viscosity

Using a solution mixed from equal parts of phenol and tetrachloroethane, the viscosity of the solution was measured at 20ºC.

B. Tensile stress

Based on the measurement method specified in ASTM D882, it was measured using 10 mm wide, 10 cm long specimens (n = 5 pieces). Additionally, the data are shown with average values for MD and TD.

C. Heat shrinkage factor

A 10 mm wide, 10 cm long specimen was left standing in an atmosphere at 150ºC under a load of about 0.4 g for 30 minutes, and the changes in size before and after the standing time were measured. And the heat shrinkage factor was calculated in terms of the percentage change in length after standing from the original length. In addition, the data are shown with average values of three pieces for the MD orientation and three pieces for the TD orientation and with average values of MD and TD.

D. Optical density

Macbeth's transmission concentration meter TD 932 was used with a transmission aperture diameter set at 3 mm, the incident light content I and transmitted light content I were measured and the transmission concentration D was calculated by the following formula, while the result is used as the optical density.

D = -log(I/I)

E. Wisdom

It was measured using the JIS L 1015 7.11 C method for white color grades (Hunter method).

F. Composite strength (heat laminability)

A sample film and a 0.21 mm thick tin-free steel plate, in a superimposed state, were fed between a metal roller heated to 240ºC and a silicone rubber roller and thermally bonded at a feeding speed of 20 m/min and a line pressure of 490 N/cm (50 kgf/cm). It was cooled with water and then it was tested in the form of a 25 nm wide test piece for 180 degree release strength under the conditions of a release speed of 10 mm/min using the Shimazu Corporation autograph. And if the release strength or film tear strength was 300 gf or greater, the product was decided as acceptable (thereafter marked with the mark "O"), while if the release strength or film tear strength was less than 300 gf, the product was decided as unacceptable (thereafter marked with the mark "X").

6. Film thickness

Thin sections were cut from a stretched film using a microtome and their corresponding thicknesses were measured using an electron microscope.

H. Wear of a can production press or die

It was evaluated how scratches were generated during the can manufacturing process. Specifically, a film was coated on a tin-free steel and a 350 ml two-piece can was deep drawn using the film side as the outside of the can. This process was carried out until 100 cans had been produced, while the press used for deep drawing was inspected for scratches during the process. If no scratches were detected, the product was decided as acceptable (afterwards marked with the "O" mark), while if some scratches, even small ones, were detected, the product was decided as unacceptable (afterwards marked with the "X" mark).

I. Composition of polyester

This was found using ¹H-MMR analysis at 300 MHz using a Varian analyzer. The content of ethylene terephthalate unit was calculated based on the proportion of terephthalic acid to the total content of acid component; the content of ethylene isophthalate unit was calculated based on the proportion of isophthalic acid to the total content of acid component; and the content of diethylene tere(iso)phthalate unit is calculated based on the proportion of diethylene glycol to the total content of glycol.

Examples 1-14 and comparative examples 1-7

The polyester resin composition forming the S layer shown in Tables 1 and 2 was melt-extruded at 280°C by a first extruder. Likewise, the polyester resin composition forming the B layer was melt-extruded at 280°C by a second extruder.

The two types of molten resins were stacked in a multi-layer press to form a three-layer structure 3/BIS having the layer thicknesses shown in Tables 1 and 2, the three-layer structures were then extruded into a strip shape by a T-die, the resulting strips were cooled by contacting with a cooling drum having a surface temperature of 18ºC; consequently, 70 to 300 µm thick unstretched strips were obtained.

The unstretched strips obtained above were fed to a tenter type simultaneous biaxial stretching machine, where they were simultaneously biaxially stretched at 90ºC with a magnification factor of 3.0 for MD and 3.3 for TD. Thereafter, with a relaxation factor of 5% for TD, a heat treatment at a temperature of 155ºC was applied for 4 seconds and they were cooled and rotated to obtain white multilayer films. with a thickness of 7 to 30 μm.

Characteristic values of the obtained films are shown in Tables 1 and 2. Table 1 (Part 1) Table 1 (Part 2) Table 2 (Part 1) Table 2 (Part 2)

In Tables 1 and 2, "IPA" means isophthalic acid and "DEG" means diethylene glycol.

From Table 1, it can be seen that Examples 1 to 14 are all very good in whiteness, concealability and heat laminability to steel strips, and no scratches made on the press during the can making process were observed.

In contrast, Comparative Example 1 showed lower heat laminability because the content of ethylene isophthalate unit in the S layer was significantly lower than 4 mol%. Comparative Example 2 had low film strength and was inferior in heat laminability because the content of ethylene isophthalate unit in the S layers was significantly larger than 27 mol%. Comparative Example 3 was low in film strength and inferior in heat laminability because the content of ethylene isophthalate unit in the B layer was significantly larger than 27 mol%. Comparative Example 4 was low in film strength and inferior in heat laminability because the content of diethylene tere(iso)phthalate unit in the S layer was significantly larger than 7.5 mol%. Comparative Example 5 was low in film strength because the content of titanium oxide in the B layer was considerably larger than 65 wt%. In Comparative Example 6, the content of titanium oxide in the B layer was considerably smaller than 16 wt%, and the content of titanium oxide in the film was also considerably smaller than 15 wt%, so that the whiteness of the film was insufficient. In Comparative Example 7, because the content of titanium oxide in the S layers was considerably larger than 25 wt%, scratches were observed in the press during the can-making process.

Claims (9)

1. A white film for laminating a metal surface, the film being a biaxially stretched film having first and second layers, at least one of the first and second layers being composed of polyester and titanium oxide, characterized, that the first and second layers are laminated in the arrangement of second layer/first layer/second layer, in which the first layer is made of a composition consisting of 80 to 40% by weight of polyester, which in turn is composed of 100 to 75 mol% of ethylene terephthalate unit and 0 to 25 mol% of ethylene isophthalate unit, these units adding up to 100 mol%, and 20 to 60% by weight of titanium oxide, which is mixed with the polyester so that the total adds up to 100% by weight, the second front and back layers are made of a composition consisting of not less than 80 to 100% by weight of polyester, which in turn is composed of 94 to 70 mol% of ethylene terephthalate unit, 5 to 25 mol% of ethylene isophthalate unit and 1 to 5 mol-% diethylene terephthalate unit and/or 1 to 5 mol-% diethylene isophthalate unit, these units add up to 100 mol-%, and 20 to 0 weight-% titanium oxide, which is mixed with the polyester so that the total adds up to 100 weight-%, the amount of titanium oxide contained in the film is 20 to 50% by weight. 2. A white film according to claim 1, in which the ratio Cb/Cs between the content Cb, in weight %, of titanium oxide in the first layer and the content Cs, in weight %, of titanium oxide in the second layers is selected such that 3 ≤ Cb/Os. 3. A white film according to claim 1, in which the ratio Cb/Cs between the content Cb, in weight %, of titanium oxide in the first layer and the content Cs, in weight %, of titanium oxide in the second layers is selected such that 5 ≤ Cb/Os. 4. A white film according to claim 1, in which the content Cb, in weight % of titanium oxide in the first layer, the thickness Tb (µm) of the first layer, the content Cs, in weight % of titanium oxide in the second layers, the thickness Ts (µm) of each of the second layers satisfy the relationship expressed by 3 ? (Cb/Os) · [Tb/(Tb+Ts)]. 5. A white film according to claim 1, in which the thickness of the first layer is 5 to 20 µm and the thickness of each second layer is 0.5 to 5 µm, the total thickness of the film is 6 to 30 µm. 6. A white film according to claim 1, in which the tensile stress is not less than 98 MPa (10 kgf/mm²), the optical density is not less than 0.3, and the whiteness is not less than 81.0. 7. A white film according to claim 1, in which the heat shrinkage factors are selected such that the transverse heat shrinkage factor ≤ 6%, and (the heat shrinkage factor in the machine direction) - (the heat shrinkage factor in the transverse direction) ≥ 3%. 8. A method of producing a white film for laminating a metal surface, the film being a biaxially stretched film comprising first and second layers, at least one of the first and second layers being composed of polyester and titanium oxide, characterized in that the first and second layers are laminated in the arrangement of second layer/first layer/second layer, in which the first layer is made of a composition consisting of 80 to 40% by weight of polyester, which in turn is composed of 100 to 75 mol% of ethylene terephthalate unit, 0 to 25 mol% of ethylene isophthalate unit, these units adding up to 100 mol%, and 20 to 60% by weight of titanium, which is mixed with the polyester so that the total adds up to 100% by weight, and the second layers, front and back, are made in a composition that is composed of 80 to 100% by weight of polyester, which in turn is composed of 94 to 70 mol% ethylene terephthalate unit, 5 to 25 mol% ethylene isophthalate unit and 1 to 5 mol% diethylene terephthalate unit and/or 1 to 5 mol% isophthalate unit, these units add up to 100 mol%, and 20 to 0% by weight titanium oxide, which is mixed with the polyester so that the total adds up to 100% by weight, the amount of titanium oxide contained in the film is 20 to 50% by weight, the procedure includes the following steps: Use of resins such that the inherent viscosities of the polyesters in the first and second layers are 0.5 or more; Production of an unstretched strip by co-extrusion of these resins in the molten state; and Stretching the resulting unstretched strip longitudinally and transversely. 9. A method according to claim 8, wherein the uneven strip is stretched longitudinally and transversely by means of a simultaneous biaxial stretching process.